Dynamic bone anchor and method of manufacturing the same

ABSTRACT

A dynamic bone anchor includes an anchor member having first and second ends and a tubular section between the ends, a longitudinal axis extending from the first to second end, an outer surface, and a bone engagement structure for engaging a bone on at least a portion of the outer surface; and a longitudinal core member having a first portion and a second portion configured to be received in the tubular section and to connected to the anchor member, with the first portion configured to be spaced apart from the anchor member and movable with respect to it. The core member is made at least partially of a nickel-titanium (Ni—Ti) based shape memory alloy such that its shape after transitioning from a martensitic to austenitic phase is configured to result in a press-fit connection between the second portion and the anchor member with the second portion in the austenitic phase.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No. 16/148,723, filed Oct. 1, 2018, which is a continuation of U.S. patent application Ser. No. 15/831,124, filed Dec. 4, 2017, now U.S. Pat. No. 10,117,695, which is a continuation of U.S. patent application Ser. No. 14/098,434, filed Dec. 5, 2013, now U.S. Pat. No. 9,861,415, which claims priority to and the benefit of U.S. Provisional Application No. 61/733,769, filed Dec. 5, 2012, in the U.S. Patent and Trademark Office, the entire content of which is incorporated herein by reference; and claims priority from European Patent Application EP 12 195 758.3, filed Dec. 5, 2012, the entire content of which is incorporated herein by reference.

BACKGROUND 1. Field

The invention relates to a dynamic bone anchor and a method of manufacturing a dynamic bone anchor. The dynamic bone anchor comprises an anchor member for anchoring to a bone or a vertebra, and a longitudinal core member provided in the anchor member, a portion of which is movable with respect to the anchor member. The core member is made, at least partially, of a material comprising a nickel-titanium (Ni—Ti) based shape memory alloy having superelastic properties. The method of manufacturing such a dynamic bone anchor makes use of the shape memory effect of the material of the core member. The dynamic bone anchor is particularly applicable in the field of dynamic bone fixation or dynamic stabilization of the spinal column.

2. Description of the Related Art

A dynamic bone anchor is known, for example, from US 2005/0154390 A1. The shaft of the bone anchor comprises an elastic or flexible section.

A further dynamic bone fixation element is known from US 2009/0157123 A1. The dynamic bone fixation element includes a bone engaging component and a load carrier engaging component. The bone engaging component includes a plurality of threads for engaging a patient's bone and a lumen. The load carrier engaging component includes a head portion for engaging a load carrier and a shaft portion that at least partially extends into the lumen. The distal end of the shaft portion is coupled to the lumen and at least a portion of an outer surface of the shaft portion is spaced away from at least a portion of an inner surface of the lumen via a gap so that the head portion can move with respect to the bone engaging component. The load carrier engaging component may be made from a high strength material, for example, a strong metal or metal alloy such as CoCrMo, CoCrMoC, CoCrNi or CoCrWNi. In a particularly preferred embodiment, the bone engaging component is made from titanium or a titanium alloy, while the load carrier engaging portion is made from cobalt chrome (CoCr).

SUMMARY

It is the object of the invention to provide an improved dynamic bone anchor that allows a head of the bone anchor to perform limited motion after anchoring the bone anchor into a bone or a vertebrae. Further, a method of manufacturing such a dynamic bone anchor shall be provided.

With the dynamic bone anchor, bone parts or vertebrae to be fixed or stabilized are able to carry out a controlled limited motion relative to each other. A longitudinal core member provided in the anchor member of the dynamic bone anchor is preferably made of Ni—Ti based shape memory alloy that is in the superelastic metallurgical state under the conditions in which the bone anchor is used in a patient.

Superelasticity, sometimes called pseudoelasticity, involves the creation of stress-induced martensite which simultaneously undergoes strain when it is formed to release the applied stress. When the applied stress is released, the thermally unstable martensite reverts to austenite, and the strain returns to zero. This process results in high elasticity in the material.

Due to the superelastic behavior of the core member, the degree of possible movement of the core member relative to the anchor member is increased compared to materials without superelasticity. The plateau in the stress-strain diagram of a Ni—Ti based shaped memory alloy shows a substantially constant stress exhibited when stress-induced martensite starts forming. This provides for an overload protection, for example during the screwing in of the dynamic bone anchor. In addition, the entire dynamic bone anchor can be designed with a relatively short length, compared to similar bone anchors made of other materials.

The head of the bone anchor can perform rotational and/or translational motions with respect to a central axis of the bone anchor.

The dynamic bone anchor can be provided as a modular system, wherein a core member can be selectively combined with anchor members of different shapes, lengths, or other properties, for example different thread types, barbs etc. This has the advantage that the characteristics of the core member that substantially define the dynamic characteristics of the whole entire dynamic bone anchor, can be utilized in different combinations of core members with anchor members.

By providing core members having different properties, such as their contour, length, etc., and combining them with suitable anchor members, various dynamic properties can be achieved.

With the method of manufacturing the bone anchor that makes use of the shape memory effect of the Ni—Ti shape memory alloy of the core member, the core member can be easily connected to anchor members of different length or other different properties. A resulting press-fit connection between the core member and the anchor member has a higher strength than conventionally generated press-fit connections.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the description of embodiments by means of the accompanying drawings. In the drawings:

FIG. 1: shows a perspective exploded view of a dynamic bone anchor according to a first embodiment.

FIG. 2: shows a perspective view of the dynamic bone anchor of FIG. 1 in an assembled state.

FIG. 3: shows a cross-sectional view of the anchor member of the dynamic bone anchor according to the first embodiment, the cross-section taken in a plane containing the anchor axis.

FIG. 4: shows a side view of the core member of the dynamic bone anchor according to the first embodiment.

FIG. 5: shows a top view of the core member shown in FIG. 4.

FIG. 6: shows a perspective view of a head of the dynamic bone anchor according to the first embodiment.

FIG. 7: shows a cross-sectional view of the head shown in FIG. 6, the cross-section taken in a plane containing the anchor axis when the head is in a non-deflected state.

FIG. 8a : shows a cross-sectional view of a step of manufacturing the dynamic bone anchor according to the first embodiment, wherein the core member is selectively combined with different anchor members.

FIG. 8b : shows a schematic view of the contour of an end portion of the core member before it is fixed to the anchor member.

FIG. 9a : shows a cross-sectional view of the assembled dynamic bone anchor according to the first embodiment, the cross-section taken in a plane containing the anchor axis.

FIG. 9b : shows a schematic view of the end portion of the core member after inserting the core member into the anchor member, and heating the core member.

FIG. 10: shows a cross-sectional view of the dynamic bone anchor according to the first embodiment illustrating a translational movement of the head relative to the anchor member.

FIG. 11: shows a cross-sectional view of the dynamic bone anchor according to the first embodiment illustrating a rotational movement of the head relative to the anchor member.

FIG. 12: shows an exemplary stress-strain diagram of the bone anchor according to one embodiment.

FIG. 13: shows a cross-sectional view of a polyaxial pedicle screw using the dynamic bone anchor according to the first embodiment as an anchoring element.

FIG. 14: shows a cross-sectional view of the dynamic bone anchor according to the first embodiment used with a bone plate to provide dynamic fixation to bone parts.

FIG. 15: shows a perspective exploded view of a dynamic bone anchor according to a second embodiment.

FIG. 16: shows a perspective view of the dynamic bone anchor according to FIG. 15.

FIG. 17: shows a cross-sectional view of the dynamic bone anchor according to FIG. 16, the cross-section taken in a plane containing the anchor axis.

FIG. 18: shows a perspective exploded view of the dynamic bone anchor according to a third embodiment.

FIG. 19: shows a perspective view of the dynamic bone anchor according to FIG. 18 in an assembled state.

FIG. 20: shows a cross-sectional view of the dynamic bone anchor according to the third embodiment, the cross-section taken in a plane containing the anchor axis.

DETAILED DESCRIPTION

As shown in FIGS. 1 and 2, a dynamic bone anchor according to a first embodiment, comprises an anchor member 1 in the form of a screw member, a core member 2 and a head 3. The core member 2 can be inserted into the anchor member 1 and connected thereto, and the head 3 can be connected to the core member 2.

As further shown in FIG. 3, the anchor member 1 comprises a first end 11, an opposite second end 12, and a longitudinal axis L extending through the first end 11 and the second end 12. The longitudinal axis L forms the central axis of the bone anchor. Adjacent to the first end 11, the anchor member 1 comprises a tubular section 13 with an opening at the first end 11. The tubular section 13 extends a distance toward the second end 12, and has an inner diameter d₁ and a length adapted to accommodate a portion of the core member 2 as described below. An end surface 13 a of the tubular section provides a stop for the insertion of the core member 2. The second end 12 of the anchor member 1 is formed as a tip portion. At least a portion of an outer surface of the anchor member 1 is provided with a bone engagement structure 14 that is configured to engage a bone or a vertebra when the anchor member 1 is used. In the embodiment shown, the bone engagement structure 14 includes a bone thread that extends over substantially the length of the anchor member 1, but it may also only extend over a smaller portion of the outer surface of the anchor member 1.

The anchor member 1 may be made of a material that has a higher modulus of elasticity than that of the material of the core member 2, meaning, the anchor member 1 is made of a stiffer material than the core member 2. In some embodiments, the anchor member 1 is made of titanium or stainless steel. The anchor member 1 can also be made of a bio-compatible polymer material, where the dimensions, such as the length and wall thickness of the anchor member are such that the anchor member does not exhibit a flexible behavior when inserted into a bone.

As shown in FIGS. 4 and 5, the core member 2 is a longitudinal member comprising a first end 21, an opposite second end 22, and a substantially rod-shaped central portion 23. The central portion 23 has a circular cross-section with an outer diameter d₂ that is smaller than the inner diameter d₁ of the tubular section 13 of the anchor member 1. Adjacent to the first end 21, there is a first connection portion 21 a, and adjacent to the second end 22, there is a second connection portion 22 a. As can be seen in particular in FIG. 5, the first and second connection portions 21 a, 22 a, respectively, have an outer contour of a square with rounded edges. A distance d₃ from one flat side to an opposite flat side of the square contour is slightly greater than the outer diameter d₂ of the central portion 23 of the core member 2 such that the second connection portion 22 a may be connected in a press-fit manner in the tubular section 13 of the anchor member 1, as explained below. The second connection portion 22 a has a length in an axial direction adapted to provide sufficient fixation within the tubular section 13. The first connection portion 21 a at the first end 21 connects with the head 3, and has a shape similar to that of the second connection portion 22 a. Between the central portion 23 and each of the first and second connection portions 21 a, 22 a, respectively, there is a transition portion each of 21 b, 22 b, respectively, with increasing outer diameter towards its respective connection portion 21 a, 22 a. The total length of the core member 2 is such that when the core member 2 is inserted into the anchor member 1, and the second end 22 of the core member 2 abuts against the end surface 13 a of the tubular section 13 of the anchor member 1, at least the first connection portion 21 a and the first transition portion 21 b of the core member 2 project out of the open first end 11 of the anchor member 1.

The core member 2 is made of a material that is based on a nickel-titanium based shape memory alloy, preferably from Nitinol. The material exhibits superelasticity. Superelasticity is present in the austenitic metallurgical state. In particular, superelasticity is present in a temperature range slightly above the stress-free martensite to austenite transition temperature, which should be the temperature range of use and includes body temperature. In one embodiment, the core member 2 is made of a nickel-titanium based shape memory alloy of the ELI (extra low interstitial) type, in particular Nitinol of the ELI type. Such a material is of high purity, and in particular, comprises less oxygen compared to other Nitinol alloys that are not of the ELI type. For example, the oxygen content may be less than 0.025 wt %, preferably equal to or less than 0.010 wt %, and more preferably equal to or less than 0.005 wt %. The material has a fatigue strength limit that can be up to two times higher than non-ELI type shape memory alloys.

The head 3 will be described with reference to FIGS. 6 and 7. The head 3 comprises a first end 31, an opposite second end 32, and a spherical-segment shaped portion 33 adjacent to the first end 31. An engagement portion 34 for engagement with a driver is at a free end surface of the first end 31. Adjacent to the spherical-segment shaped portion 33, there is a cylindrical neck portion 35 with a cylindrical recess 36 for accommodating the first connection portion 21 a of the core member 2. The length and an inner diameter of the recess 36 is such that the first connection portion 21 a can be accommodated therein with a press-fit connection.

FIGS. 8a to 9b illustrate steps of manufacturing the dynamic bone anchor according to the first embodiment. First, the core member 2 may be pre-assembled with the head 3 by connecting the first connection portion 21 a to the cylindrical recess 36 of the head 3 in a press-fit manner. The second connection portion 22 a may be previously or already shaped in its final shape, as shown in FIGS. 4 and 5. Then, at least the second connection portion 22 a of the core member may be cooled down below a martensite finish temperature Mf, resulting in a phase transition of the material from austenite into martensite.

As shown in FIG. 8a , a modular system may be provided comprising at least two anchor members 1, 1′, that differ, for example, in their shaft lengths. The core member 2 may be selectively introduced into the tubular section 13 of one of the at least two anchor members 1, 1′.

Referring to FIG. 9a , the pre-assembled core member 2 with the head 3 is introduced into the tubular section 13 of one of the anchor members 1, 1′, until the second end 22 of the core member 2 abuts against the end surface 13 a of the tubular section 13. Thereby, the second connection portion 22 a of the core member 2 is deformed such that, for example, the flat sides are impressed so that they have a smaller distance from each other than in the original shape, as depicted in FIG. 8b . Accordingly, the second connection portion 22 a of the core member 2 can be introduced into the tubular section 13 of the anchor member 1. Due to the ability of the martensite phase to deform, the insertion can be achieved with low force and little abrasion.

In a next step, heating the second connection portion 22 a above the austenite finish temperature A_(f) effects a phase transition from martensite to austenite and a change of shape of the second connection portion 22 a back to its original shape with the rounded square contour as shown in FIG. 9b resulting from the shape memory of the material. Hence, the manufacturing process uses the shape memory behavior of the core member 2. By means of this procedure and the square shape of the second connection portion 22 a, a particularly strong press-fit connection can be achieved through a distortion-fit connection using the shape memory effect, which is stronger than a press-fit connection based on conventional machining techniques.

It should be noted that the connection between the core member 2 and the head 3 can also be made in the same manner described above.

Referring to FIG. 10, in the assembled state, there is a gap 37 between the second end 32 of the head 3 and the first end 11 of the anchor member 1. Also, there is a gap 38 between the central portion 23 of the core member 2 and a wall of the tubular section 13. These gaps 37, 38 allow the head 3 to perform translational movement with respect to the anchor member 1 in a direction substantially perpendicular to the anchor axis L. The extent of deflection from the central axis L of the bone anchor depends on the elasticity of the material of the core member 2 and the size of the gaps 37, 38, which depend on the thickness and length of the core member 2. A translation movement may occur when the deflection of the core member 2 is mostly in the region of the first connection portion 21 a.

Referring to FIG. 11, also a rotational movement of the center point of the head 3 around the anchor axis L is possible. For the rotational movement, the deflection of the core member takes place almost over the whole length of the central portion 23 and the first connection portion 21 a. Due to the superelasticity of the material of the core member 2 a deflection of the core member 2 out of the anchor axis is possible with a core member that is shorter compared to a core member made of another metallic material.

In use, the dynamic bone anchor is inserted into a bone part or a vertebra. Because the core member 2 is in the austenitic metallurgical state and in the conditions of use, the core member 2 has superelastic characteristics. In the stress-strain diagram of the bone anchor shown in FIG. 12, a stress-strain plateau is shown. Because of the plateau, the force acting onto the head 3 during screwing-in of the bone anchor remains constant over a certain range such that an overloading of the anchor head 3 may not occur.

In the anchored state, the head 3 is capable of performing a limited motion. The motion is constrained by the abutment of the core member 2 against the inner surface of the tubular section 13 of the anchor member 1.

A first application of the bone anchor together with a stabilization device is shown in FIG. 13. The bone anchor according to the first embodiment is coupled to a receiving part 4 to form a polyaxial bone anchor. The receiving part 4 is substantially cylindrical and comprises a top end 41, a bottom end 42, and a coaxial bore 43 extending from the top end 41 to a distance from the bottom end 42. The bore 43 narrows towards the bottom end 42 and has an opening 44 near the bottom end 42. Near the opening 44, a seat 45 is provided for pivotably receiving the head 3. A U-shaped recess extends from the top end 41 to a distance from the top end 41 for receiving a stabilization rod 5. The U-shaped recess creates two free legs 46, 47, which have an internal thread 48 for cooperating with a locking member 6, such as a set screw. Furthermore, a pressure member 7 is provided that exerts pressure onto the head 3 such that the head 3 can be locked in a certain angular position by tightening the locking member 6. The bone anchor may be used with other designs of receiving parts and polyaxial bone screws. Also, the head 3 of the core member 2 may be designed such that it comprises a section for receiving a rod and for receiving a locking member to fix the rod, as known from other monoaxial bone screws. In use, at least two polyaxial bone anchors may be used and connected via the rod 5. Once the anchor members 1, 1′ are inserted into the bone parts or adjacent vertebrae, the heads 3, 3′ can perform a limited motion with respect to the anchor members 1, 1′, respectively. Once each head 3, 3′ is locked in its respective receiving part 4, the bone anchor provides for a dynamic stabilization that allows small movements of the bone parts with respect to each other, or small movements of a motion segment of the spinal column.

A second example of application of the bone anchor according to another embodiment is shown in FIG. 14. In this embodiment, the dynamic bone anchors according to the first embodiment are used together with a bone plate 9 comprising holes 9 a, 9 a′ with seat portions 9 b, 9 b′ for receiving the heads 3, 3′ of two bone anchors 1, 1′, respectively. The two bone anchors 1, 1′ are inserted in adjacent bone parts 101, 101′ and the bone plate 9 bridges at least a portion of a fracture site 102. In a specific application, a distance between the central anchor axes C of the two holes 9 a, 9 a′ that accommodate the heads 3, 3′ of the bone anchors, respectively, is slightly smaller than the distance between the longitudinal axes L of the anchor members 1, 1′. Because the core members 2, 2′ with the heads 3, 3′ can perform a limited motion in a direction transverse to the longitudinal axis L, the bone parts 101, 101′ can be drawn together at the fracture site 102 as shown by the arrows in FIG. 14.

Referring to FIGS. 15 to 17, a second embodiment of the dynamic bone anchor is described. The dynamic bone anchor according to the second embodiment differs from the dynamic bone anchor according to the first embodiment in that the anchor member 1″ is formed completely as a tubular member 13″. That means, the anchor member 1″ is open at the first end 11 and at the second end 12. The core member 2″ includes a tip 24 between the second connection portion 22 a and the second end 22. The second connection portion 22 a is configured to be fixed in the previously described manner via a distortion-fit connection using the shape memory effect to the portion adjacent to the second end 12 of the anchor member 1″. The tip 24 of the core member 2″ projects out of the open second end 12 of the anchor 1″. The tip 24 may have a smooth surface or may have further features, such as a self tapping structure, barbs, or a roughened surface, etc. All other parts of the dynamic bone anchor according to the second embodiment are similar to the first embodiment and the description thereof is not repeated.

Referring to FIGS. 18 to 20, a third embodiment of the dynamic bone anchor will be described. The bone anchor according to third embodiment differs from the bone anchor according to the second embodiment in that the core member 2″′ comprises a head 3″′ similar to the head 3 of the first embodiment at the first end 21, wherein the head 3″′ is formed integrally with the central portion 23 of the core member 2″′. Hence, the core member 2″′ with head 3″′ is a monolithic piece, and the entire dynamic bone anchor comprises only two pieces. The outer contour of the head 3″′ is similar to the head 3 according to the first embodiment as previously described.

Further adaptations or modifications of the dynamic bone anchor described in the embodiments can be accomplished by one of ordinary skill in the art without departing from the scope of the invention. For example, the head may have any other shape suitable for connecting it to other stabilization devices such as bone plates, receiving parts for accommodating stabilization rods etc. The head may even be omitted if a free end of the core member is suitable for connection to another device. In such a case, the free end of the core member may comprise an engagement portion for a driver. In both cases, with or without a head or a head portion, the engagement portion of the bone anchor for engagement with a tool is at a movable end of the core member.

Any kind of tip may be provided. The tip shown in the embodiments may even be omitted. For example, the hollow tubular anchor member according to the second and third embodiments, may have prongs at the second end.

The bone engagement structure may be a bone thread of any type suitable for engaging the bone, or may be accomplished by barbs or may even be only a roughened surface.

The embodiments may also be combined among each other, only as an example for such a combination, the anchor member of the first embodiment may include a core member having an integrally formed head, as described in the third embodiment.

While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is instead intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof. 

What is claimed is:
 1. A dynamic bone anchor comprising: an anchor member having a first end, a second end, and a tubular section between the first end and the second end having an inner surface defining a bore, the anchor member defining a longitudinal axis extending from the first end to the second end, and comprising an outer surface and a bone engagement structure on at least a portion of the outer surface for engaging a bone; and a core member at least partially insertable into the tubular section, and having a connection portion at or near an end of the core member that is configured to engage the inner surface of the tubular section to maintain an axial position of the connection portion relative to the tubular section when the core member is inserted into the tubular section, wherein a cross-sectional shape of the bore in a first plane transverse to the longitudinal axis is different from a cross-sectional shape of the connection portion in a plane transverse to a central axis of the core member, and wherein a minimum width of the connection portion measured through the central axis is greater than a width of a portion of the core member adjacent to the connection portion measured through the central axis.
 2. The dynamic bone anchor of claim 1, wherein the cross-sectional shape of the connection portion is non-circular.
 3. The dynamic bone anchor of claim 1, wherein when the core member is not inserted into the tubular section, a width of the connection portion is configured to be greater than a width of the bore.
 4. The dynamic bone anchor of claim 1, wherein the connection portion is configured to be deformed when inserted into the tubular section.
 5. The dynamic bone anchor of claim 4, wherein at least the connection portion of the core member comprises a first material comprising a nickel-titanium (Ni—Ti) based shape memory alloy.
 6. The dynamic bone anchor of claim 5, wherein the Ni—Ti based shape memory alloy is Nitinol of the Extra Low Interstitial (ELI) type.
 7. The dynamic bone anchor of claim 5, wherein the anchor member comprises a second material different from the first material.
 8. The dynamic bone anchor of claim 7, wherein the second material has a greater modulus of elasticity than that of the first material.
 9. The dynamic bone anchor of claim 1, wherein the core member forms a distal tip of the bone anchor.
 10. The dynamic bone anchor of claim 9, wherein at least the tip comprises a Nitinol of the ELI type to enhance fatigue strength.
 11. The dynamic bone anchor of claim 9, wherein the tip formed by the core member is configured to protrude from the second end of the anchor member when the core member is fully inserted into the tubular section from the first end of the anchor member.
 12. A dynamic bone anchor comprising: an anchor member having a first end, a second end, and a tubular section between the first end and the second end and having an inner surface defining a bore, the anchor member defining a longitudinal axis extending from the first end to the second end, and comprising an outer surface and a bone engagement structure on at least a portion of the outer surface for engaging a bone; and a core member at least partially insertable into the tubular section, and having a connection portion at or near an end of the core member that is configured to engage the inner surface of the tubular section to maintain an axial position of the connection portion relative to the tubular section when the core member is inserted into the tubular section, wherein a cross-sectional shape of the connection portion in a plane transverse to a central axis of the core member is non-circular, and wherein a minimum width of the connection portion measured through the central axis is greater than a width of a portion of the core member adjacent to the connection portion measured through the central axis.
 13. The dynamic bone anchor of claim 12, wherein a cross-sectional shape of the bore is circular.
 14. The dynamic bone anchor of claim 12, wherein when the core member is not inserted into the tubular section, a width of the connection portion is configured to be greater than a width of the bore.
 15. The dynamic bone anchor of claim 14, wherein the connection portion is configured to be deformed when inserted into the tubular section.
 16. The dynamic bone anchor of claim 15, wherein when the connection portion is inside the tubular section, the connection portion is configured to expand to apply an outward force on the inner surface of the tubular section.
 17. The dynamic bone anchor of claim 12, wherein at least the connection portion of the core member comprises Nitinol.
 18. The dynamic bone anchor of claim 17, wherein the Nitinol is of the ELI type.
 19. The dynamic bone anchor of claim 12, wherein the core member forms a distal tip of the bone anchor.
 20. The dynamic bone anchor of claim 19, wherein at least the tip comprises a Nitinol of the ELI type to enhance fatigue strength. 